Chrome alum

[Kirk Keyes]

"Potassium Chroum"

I like that. Hard to say, but still cool.

You should submit that to IUPAC.

[paul_c5x4]

You should not use Alum (white powder sold in grocery stores and for film and paper hardeners) for hardening emulsions that are going to be coated. You can use it in food.

I've seen many dilutions for chrome alum suggested for hardening gelatin - Anything from 0.025% and upwards. What would you (PE) recommend for general usage ?

[Photo Engineer]

Paul;

Chrome alum can be used to harden both raw emulsions before coating and as a hardener during processing. This solution is quite toxic. Handle with care.

For coating, I make a 10% solution in water and add about 5 ml / 200 ml of 10% gelatin. The hardness is about the same over that range, but the length of time needed to achieve good hardness varies from 24 hours to about 96 hours.

PE

[hrst]

Additional question; I've got an impression that chrome alum works by a slightly different mechanism than formaldehyde or glyoxal.

So, can I get even more hardening if I use both chrome alum and glyoxal in the same emulsion, if they both work at the same time by their own mechanisms?

Not sure though if I need more hardening.

[Photo Engineer]

Yes, the Chrome Alum and Alum hardeners act by forming an ionic bond with gelatin. The Aluminum based Alum can be reversed and the film or paper can become softer, but I am not sure about Chrome Alum due to its strength. In addition, Chrome Alum forms a bond with glass plates.

Formaldehyde forms a covalent bond with gelatin and once formed it is very difficult to reverse! So the bonds are totally different in nature and in effect. As you go from formalin to glyoxal to succinaldehyde to glutaraldehyde, you also form a "longer" bond length which changes the characteristic of the hardening.

PE